In a common design the vehicle seat has a rigid frame structure, often with adjustable reclining angle backrest, which structure is adapted to absorb the weight of the occupant, which is seated in a normal upright position on the seat cushion while the occupant's torso is supported from behind by the backrest cushion.
In this document it is assumed that the occupant is facing forward when seated and the collision is considered to be a frontal collision. The skilled person will appreciate that the seat can be advantageous in case of a rear collision as well. Also the skilled person will appreciate that the seat could be arranged facing rearward, although a forward facing arrangement is preferred.
The term vehicle at least includes automobiles, vans, (public transport) buses, mini-buses, coaches, trucks, lorries, race-cars, trains, aeroplanes.
In most automobiles a three-point seat belt is common to retain the occupant in the seat upon a collision. Commonly the three-point seat belt system has its shoulder guide member for the seat belt integrated in a pillar of the car body.
For vans, buses (including mini-buses) and coaches it is common to provide a lap belt seat belt system even though occupant safety would greatly benefit from a three-point safety belt system. However most of the presently known seat designs, especially of the passengers seats in vans, (mini-)buses and coaches, are unsuitable for integration of a three-point safety belt system. In particular the backrest and the attachment thereof to the lower part of the frame structure of the passenger seats in these vehicles is commonly unable to withstand the forces that would act on the backrest in the region of the shoulder seat belt guide in case of a collision. In Australia legislation has led to development of coach seats with integrated three-point seat belt systems, such as disclosed in patent application AU2007200954 of StyleRide Pty.
It is noted that for bus and truck drivers complex, heavy and quite expensive seats have been developed, wherein a three-point safety belt system is integrated in the seat. An example is the Recaro commercial vehicle driver seat traded under the name Profi Class Sigus. In said known seat the seat belt refractor is mounted in the upper part of the backrest.
In case of a collision these seats with integrated seat belt system are designed to maintain the occupant as good as possible in his/her normal position, and the seat cushion and backrest are generally designed to maintain their original position. Upon collision the seat belt exerts a large load on the upper part of the backrest, and it will be understood that in this known seat the backrest and its attachment to the lower part of the seat structure are designed very robust to absorb the large momentum on the backrest and its attachment.
Depending on the requirements imposed on such a seat with integrated seat belt, primarily on the g-force requirement during collision, a very robust design of the rigid frame structure of the seat will be needed in order to absorb the forces of the seat belt upon collision. This is likely to lead to a significant increase of the weight of the seat when compared to presently used passenger seat. E.g. in a bus with about 50 seats such weight increase would be highly problematic. Also the floor and/or other vehicle body element (e.g. the side wall as in a coach) on which the seats are mounted would have to be designed with extra strength, both to support the seat and to absorb the forces upon collision and/or heavy breaking, again adding to the total weight of the vehicle. For an aeroplane this issue would even be more problematic.
Another problem associated with a three-point seat belt system is the problem of “seat belt slack”. Seat belt slack leads to an undesirable displacement of the occupant with respect to the seat. In a collision the occupant will accelerate forward and then be abruptly stopped by the seat belt, leading to undesirable loads on the body. Obviously the occupant could reduce this effect by tightening the seat belt himself, but it is found that most occupants do not so even in automobiles, and for public transport vehicles it is expected that even less occupants would do so.
In order to reduce seat belt slack “pre-tensioners” have been developed, often integrated with a seat belt anchoring member or seat belt lock member. Many pretensioners are pyrotechnic devices, which are triggered upon an actual collision, upon command of a suitable sensor, and then tension the seat belt. Also a pretensioning seat belt retractor is known, which is also governed by a suitable sensor, e.g. based upon the deceleration caused by the braking of the vehicle by the driver. For application in buses etc, with many seats such pretensioners are likely to be too expensive, both in terms of acquisition and maintenance.